Modulation types such as OFDM, QPSK and QAM can be used for terrestrial transmission of digital television and broadcast radio signals (referred to in general as broadcast radio signals in the following text). Examples of such broadcast radio signals include DVB (digital video broadcast), HDTV-T (hierarchical digital television transmission) and DAB (digital audio broadcast). The fundamental principles of the DVB system are specified in ETS 300 744.
The data in digital broadcast radio signals are arranged in two-dimensional (time and frequency, called "temporal-spec-tral" in the following text) frames which have a time duration of TF and, in the case of ETS 300 744, comprise 68 OFDM symbols. Four frames form a superframe. Various transmission modes may be used in the transmission systems for digital audio or video signals mentioned above. In the case of ETS 300 744, symbols of duration Ts are in each case formed from 1705 carriers (2K mode) or from in each case 6817 carriers (8K mode) at different frequencies.
The 2K mode is particularly suitable for individual transmitters and small SFN networks (single frequency networks) with limited distances between transmitters.
The 8K mode can be used for individual transmitters and for small and large SFN networks.
The symbols have a time duration of Ts with a wanted part of duration Tu and a guard interval of duration .DELTA.. The guard interval is formed by cyclic continuation of the wanted part, and is arranged before the latter, in time. All symbols contain data and reference information. Each symbol may be regarded as a group of cells, one cell corresponding to each carrier.
Apart from the actual video, audio or other data, the frames contain scattered pilot cells (scattered pilots) continuous pilot signals and TPS carriers or pilots (transmission parameter signaling). These are described, for example, in Sections 4.4 to 4.6 of ETS 300 744, March 1997.
The pilot cells or carriers contain reference information whose transmitted value is known by the receiver. The continuous pilot signals may coincide with a scattered pilot cell, for example in every fourth symbol. The value or contents of the scattered and continuous pilot signals are derived, for example, from a pseudo-random binary sequence W.sub.k for each of the transmitted carriers k. The sequence W.sub.k may also define the start phase of the TPS carrier information. The pilot cells or carriers may be used at the receiver end for frame synchronization, frequency synchronization, time synchronization, channel estimation and transmission mode identification. The receiver manufacturer is free to chose whether and how these options are used at the receiver end.
EP-A-0 786 889 describes a corresponding system for use with DAB.